The present invention relates to a process for transmitting messages from an infrastructure of a radio communication network to mobile stations, as well as the equipment for implementing this process.
The invention applies to the broadcasting, by a base station of the infrastructure, of service messages destined a priori for all the mobile stations situated within the zone of coverage of the base station, each mobile station being able to decide whether or not to receive these service messages. The relevant broadcasting channels can in particular be channels for monitoring radio links if the service messages convey signals for setting up and managing the communications, channels for broadcasting service messages of which the users are intended to be made aware, or else mixed channels on which both these types of service messages will be broadcast.
To illustrate the problems raised by the broadcasting of such messages, in what follows we consider the example of the GSM ("Groupe Special Mobiles") European cellular digital radio communication system, for which there is a message broadcasting service, termed "SMS-CB" standing for "Short Message Service Cell Broadcast". When this service is provided, a particular radio channel is used in each cell and a message is broadcast roughly every two seconds on this channel. A mobile station equipped for this service reads the messages on this channel and displays them on a screen. Typically, the messages are rebroadcast cyclically.
The applications of this service are diverse. Those exploiting the network may use it for information relating to other services, as for example the tariff zones. It may also be used by external service providers, for example to broadcast travel information or weather forecasts.
The service messages are made up of 88 bytes sent in four successive blocks of equal length, the first of which contains four bytes for identifying the message, including 2 bytes, namely 16 bits, which consist of a message category cue. Each block can be received independently by the mobile stations, in the sense that reception of a block may be correct whether the other blocks have or have not been correctly received, and that the mobile station can decide whether or not to receive each individual block.
In the current state of the public specifications for the system, the mobile stations have to receive at least the first blocks of all the messages, failing prior information as to their contents, even if the user is only interested in certain of these messages. This tends to shorten the electrical endurance of the mobile stations, each reception consuming energy. Now, the endurance of portable mobile stations, which are light in weight and therefore have a small battery, is a point of importance to users.
To remedy this drawback, studies are in progress into reducing the number of receptions required, by sending information in advance. The objective is to provide indications in advance enabling the mobile station to know whether or not it is interested in a given message. Cases in which it is not interested are for example messages of a category of no interest to the user (the latter may program his apparatus to, for example, indicate to it that he is not interested in travel information). Another case is that of messages already received. Thus, the messages are rebroadcast regularly, there being many which are not modified for hours or days.
It has in particular been proposed to broadcast, on the same radio channel, schedule messages each giving information about a number of service messages which will be broadcast consecutively after the schedule message. The period for which the service messages catalogued in the schedule message are broadcast is called the schedule period. The information contained in the schedule message includes the categories of the various messages to be broadcast during the schedule period, so that a mobile station which has been programmed to ignore certain categories of message, may refrain from receiving the messages belonging to these categories without even having had to receive their first blocks. The service messages which will be broadcast several times during the schedule period are also identified within the schedule message, so that the mobile station may decide to receive these messages just once during the schedule period if this reception takes place without error. If a transmission error is detected in one of the blocks of the message received during the first broadcast, the mobile station can still decide to receive this block during the repetition of the message.
Let us consider for example the case in which the broadcasts to occur during the schedule period are those illustrated in table I. The content of the schedule message is then as indicated in table II.
After the header, the information relating to the broadcasts of messages are ranked in the sequence in which these broadcasts will occur. For each broadcast, the first bit indicates whether this is a first broadcast of a message during the schedule period (0) or a repetition (1), and the remainder of the field designates the category of the message, coded on 15 bits so as to obtain fields of 2 bytes (in the case of GSM, this entails omitting one bit from the complete category cue which contains 16 bits; it is therefore not impossible that there may be a few cases ambiguous to the mobile stations).
TABLE I ______________________________________ Broadcasting Message to be Category of sequence broadcast the message ______________________________________ 1 M1 c1 2 M2 c2 3 M3 c1 4 M3 c1 5 M2 c2 6 M1 c1 ______________________________________
TABLE II ______________________________________ Header (essentially the number of broadcasts to occur during the schedule period, here: 6) ______________________________________ 0 c1 0 c2 0 c1 1 c1 1 c2 1 c1 ______________________________________
If a mobile station receiving this schedule message is interested only in category c1, it may refrain from receiving the messages in the 2.sup.nd and 5.sup.th positions. However, it must read at least the first blocks of the messages in the 1.sup.st and 3.sup.rd positions. If, for example, the message M3 has been correctly received by the mobile station before the broadcasting of the schedule message, the mobile station must receive this same message again, at least its first block, in a superfluous manner in the 3.sup.rd position of the current period. Moreover, if the mobile station misses reception of the first block of this message in the 3.sup.rd position, it will also have to receive in a superfluous manner at least the first blocks of the messages broadcast in the 4.sup.th and 6.sup.th positions.
The schedule messages which have been proposed are therefore not optimal in terms of economy of the energy resources of the mobile stations. A main objective of the invention is to afford a solution which performs better in this respect.